The present invention relates to a gray scale display reference voltage generating circuit for use in an LCD(Liquid Crystal Display) drive unit or the like and to an LCD drive unit employing the circuit.
The above-mentioned gray scale display reference voltage generating circuit is a circuit for forming intermediate voltages in between two voltages. For example, in an LCD drive section or the like in an active matrix system LCD device, intermediate voltages are formed by resistance division. Then, resistors for resistance division have a resistance ratio called the gamma (xcex3) correction, and according to this resistance ratio, the optical characteristics of a liquid crystal material are corrected to provide more natural gray scale display.
Reference will be made below to the construction of an LCD device provided with the aforementioned gray scale display reference voltage generating circuit, the construction of a TFT (Thin Film Transistor) type LCD panel in the LCD device, an LCD drive waveform and a source driver for the LCD panel.
FIG. 6 shows a block diagram of a TFT system LCD device as a representative example of the active matrix system. This LCD device is separated into an LCD section and an LCD drive circuit (LCD drive section) for driving the LCD section. The above-mentioned LCD section has a TFT system LCD panel 1. Then, the LCD panel 1 is internally provided with an LCD element (not shown) and an opposite electrode (common electrode) 2 described in detail later.
On the other hand, the LCD drive circuit includes a source driver assembly 3, a gate driver assembly 4 and a controller 5, which are constructed of ICs (Integrated Circuits) as well as an LCD drive power supply 6. The controller 5 inputs display data D and a control signal S1 to the source driver assembly 3 and inputs a vertical synchronization signal S2 to the gate driver assembly 4. Further, a horizontal synchronization signal is inputted to the source driver assembly 3 and the gate driver assembly 4.
In the above-mentioned construction, the externally inputted display data is inputted as the display data D that is a digital signal to the source driver assembly 3 via the controller 5. Then, the source driver assembly 3 time-sharingly divides the inputted display data D, latches the data into a first source driver through an n-th source driver and thereafter subjects the latched data to D/A (Digital-to-Analog) conversion in synchronization with the horizontal synchronization signal inputted from the controller 5. Then, a gray scale display use analog voltage (referred to as a gray scale display voltage hereinafter) formed by subjecting the time-sharingly divided display data D to D/A conversion is outputted to the corresponding LCD element inside the LCD panel 1 via a source signal line (not shown).
FIG. 7 shows the construction of the LCD panel 1. The LCD panel 1 has pixel electrodes 11, pixel capacitors 12, TFTs 13 for controlling the turning-on and -off of a voltage to be applied to the pixel electrodes 11, source signal lines 14, gate signal lines 15 and an opposite electrode 16 (corresponding to the opposite electrode 2 of FIG. 6). In this case, an LCD element A of one pixel is constructed of the pixel electrode 11, the pixel capacitor 12 and the TFT 13.
The aforementioned gray scale display voltages corresponding to the brightnesses of the pixels to be used for display are applied from the source driver assembly 3 of FIG. 6 to the source signal lines 14. On the other hand, scanning signals for successively turning on the TFTs 13 arranged in the direction of column is applied from the gate driver assembly 4 to the gate signal lines 15. Then, via each TFT 13 in the ON-state, the gray scale display voltages of the source signal lines 14 are applied to the pixel electrodes 11 connected to the drains of the TFTs 13 and elective charges are accumulated in the pixel capacitors 12 provided between the pixel electrode and the opposite electrode 16. Thus, the light transmittance of the liquid crystals is changed in accordance with the gray scale display voltage, executing pixel display.
FIG. 8 and FIG. 9 show an example of an LCD drive waveform. In FIG. 8 and FIG. 9, the reference numerals 21 and 25 denote the drive waveforms of one source driver of the source driver assembly 3, while the reference numerals 22 and 26 denote the drive waveforms of one gate driver of the gate driver assembly 4. The reference numerals 23 and 27 denote the potential of the opposite electrode 16, while the reference numerals 24 and 28 denote the voltage waveforms of the pixel electrode 11. In this case, the voltage applied to the liquid crystal material is a potential difference between the pixel electrode 11 and the opposite electrode 16 and is indicated by the hatching in the figures.
For example, in the case of FIG. 8, the TFT 13 is turned on only when the level of the drive waveform 22 of the gate driver is at H-level, by which a voltage of the difference between the drive waveform 21 of the source driver and the potential 23 of the opposite electrode 16 is applied to the pixel electrode 11. Subsequently, the level of the drive waveform 22 of the gate driver comes to be at L-level, by which the TFT 13 is turned off. In this case, due to the provision of the pixel capacitor 12 for the pixel, the aforementioned voltage is retained.
The same thing can be said for the case of FIG. 9. It is to be noted that FIG. 8 and FIG. 9 respectively show the cases where different voltages are applied to the liquid crystal material. In the case of FIG. 8, the application voltage is higher than that of FIG. 9. As described above, by varying the voltage applied as an analog voltage to the liquid crystal material, the light transmittance of the liquid crystals is varied in an analog manner, providing multilevel gray scale display. It is to be noted that the number of levels of gray that can be provided for display depends on the number of analog voltages to be selectively applied to the liquid crystal material.
FIG. 10 shows a block diagram of an example of the n-th source driver constituting part of the source driver assembly 3 of FIG. 6. The display data D of the inputted digital signal has display data (DR, DG, DB) of R (red), G (green) and B (blue) . Then, the display data D is once latched into an input latch circuit 31 and thereafter time-sharingly stored into a sampling memory 33 in accordance with the operation of a shift register 32 that effects shifting by a start pulse SP and a clock CK supplied from the controller 5. Subsequently, the data are collectively transferred to a hold memory 34 on the basis of the horizontal synchronization signal (not shown) supplied from the controller 5. It is to be noted that the reference letter S denotes a cascade output.
A gray scale display reference voltage generating circuit 39 generates a reference voltage at each level on the basis of a voltage VR supplied from an external reference voltage generating circuit (corresponding to the LCD drive power supply 6 of FIG. 6). The data in the hold memory 34 is transmitted to a D/A converter circuit (Digital-to-Analog converter circuit) 36 via a level shifter circuit 35 and converted into an analog voltage on the basis of the reference voltage at each level from the gray scale display reference voltage generating circuit 39. Then, the analog voltage is outputted as the aforementioned gray scale display voltage from an LCD drive voltage output terminal 38 to the source signal lines 14 of the LCD elements A by an output circuit 37. That is, the number of levels of the reference voltages becomes the number of levels of gray that can be provided for display.
FIG. 11 shows the construction of the gray scale display reference voltage generating circuit 39 generating intermediate voltages for outputting a plurality of reference voltages as described above. It is to be noted that the gray scale display reference voltage generating circuit 39 of FIG. 11 generates 64 levels of reference voltages.
This gray scale display reference voltage generating circuit 39 is constructed of nine gray scale voltage input terminals indicated by V0, V8, V16, V24, V32, V40, V48, V56 and V64, resistor elements R0 through R7 having a resistance ratio for gamma correction and a total of 64 resistors (not shown) that are in groups of eight serially connected across both terminals of the resistor elements R0 through R7. As described above, the resistance ratio called the gamma correction is built into the source driver, providing the LCD drive output voltage for conversion into the gray scale display voltage with a line graph characteristic. Therefore, by correcting the optical characteristics of the liquid crystal material by the aforementioned resistance ratio, there can be provided natural gray scale display conforming to the optical characteristics of the liquid crystal material. An example of the LCD drive output voltage characteristic of the conventional gray scale display reference voltage generating circuit 39 is shown in FIG. 12.
However, the aforementioned conventional gray scale display reference voltage generating circuit has the problems as follows. That is, the optimum gamma correction characteristic (the line graph characteristic of the LCD drive output voltage shown in FIG. 12) varies depending on the type of the liquid crystal material and the number of pixels of the LCD panel and varies every LCD module. Then, the resistance division ratio of the built-in gray scale display reference voltage generating circuit 39 of the source driver is determined during the design phase of the source driver. Therefore, when changing the gamma correction characteristic according to the type of the adopted liquid crystal material and the number of pixels of the LCD panel, there is the problem that the source driver is required to be remade on all such occasions.
There can be considered a method for providing a reference voltage adjusting means for adjusting the plurality of gray scale input voltages supplied from the external reference voltage generating circuit to the gray scale voltage input terminals V0 through V64, and adjusting the intermediate voltages to be supplied to the gray scale voltage. input terminals V0 through V64 by the reference voltage adjusting means. However, the provision of the reference voltage adjusting means increases the number of terminals and the circuit scale, leading to an increase in manufacturing cost.
Accordingly, the object of the present invention is to provide a gray scale display reference voltage generating circuit capable of changing the gamma correction characteristic according to the liquid crystal material and LCD panel characteristics without increasing the manufacturing cost and an LCD drive unit employing the circuit.
In order to achieve the aforementioned object, according to the first aspect of the present invention, there is provided a gray scale display reference voltage generating circuit for generating reference voltages for gray scale display used in converting display data from a digital form into an analog form, comprising:
a reference voltage producing circuit for producing reference voltages of a plurality of levels; and
adjustment circuits for adjusting the reference voltages on the basis of external adjustment data.
According to the above-mentioned construction, each of the plurality of levels of reference voltages produced by the reference voltage producing circuit is adjusted by the adjustment circuits on the basis of the external adjustment data. Therefore, even after the LCD drive unit is once mounted with the gray scale display reference voltage generating circuit, the reference voltages can be simply adjusted in accordance with the liquid crystal material and the LCD panel characteristics by externally supplying the adjustment data without remaking the LCD drive unit.
In one embodiment, each of the adjustment circuits comprises:
an input terminal of the reference voltage; an input terminal of the adjustment data; an output terminal of an adjusted voltage; and an adjusted voltage generating circuit that generates a voltage higher than the reference voltage or a voltage lower than the reference voltage according to the adjustment data and outputs the resulting voltage as the adjusted voltage.
According to the above-mentioned construction, the voltage higher or lower than the reference voltage inputted from the input terminal is generated in accordance with the adjustment data by the adjusted voltage generating circuit and outputted as an adjusted voltage from the output terminal.
Furthermore, in one embodiment, the adjusted voltage generating circuit comprises:
a potential difference generating circuit for generating a potential difference according to the adjustment data; and
a sum voltage output circuit for outputting a sum voltage of the reference voltage and the potential difference,
the sum voltage from the sum voltage output circuit being outputted as the adjusted voltage.
According to the above-mentioned construction, the potential difference corresponding to the adjustment data is generated by the potential difference generating circuit. Then, the sum voltage of the potential difference and the reference voltage is generated by the sum voltage output circuit and outputted as the adjusted voltage.
Furthermore, in one embodiment, the adjusted voltage generating circuit comprises:
a resistor element that is provided between the input terminal and the output terminal and generates a potential difference corresponding to a value of current flowing through the resistor element;
at least one constant current source; and
at least one switch element that is turned on and off on the basis of the adjustment data and interposed between the constant current source and the resistor element,
the potential difference being varied by varying the value of current flowing through the resistor element by turning-on and-off control of each switch element on the basis of the adjustment data.
According to the above-mentioned construction, when each switch element interposed between the constant current source and the resistor element is controlled to be turned on and off on the basis of the adjustment data, the value of the current flowing through the resistor element interposed between the input terminal and the output terminal is changed. As a result, the potential difference generated across both terminals of the resistor element is changed to adjust the reference voltage by the quantity of adjustment corresponding to the adjustment data and outputted as the adjusted voltage.
Furthermore, one embodiment comprises a buffer amplifier interposed between the resistor element and the output terminal.
According to the above-mentioned construction, an output impedance is reduced by the buffer amplifier interposed between the resistor element and the output terminal, by which an output current is stably taken out of the output terminal.
Furthermore, in one embodiment, the constant current source generates a current weighted with 2(nxe2x88x921) assuming that n is a positive integer, and
the adjustment data is multi-bit digital data of binary digits expressed by two""s-complement representation.
According to the above-mentioned construction, by setting the bit number of the adjustment data to n, the adjustment data and the weight of the constant current source can be made to correspond to each other, and a potential difference of a multiple corresponding to the adjustment data is generated across both terminals of the resistor element.
Furthermore, in one embodiment, the constant current source is comprised of at least one first constant current source for flowing a current into the resistor element and at least one second constant current source for flowing a current out of the resistor element, and
at least one of the switch elements is capable of flowing a current from the first constant current source into the resistor element and the other of the switch elements is capable of flowing a current out of the resistor element to the second constant current source.
According to the above-mentioned construction, if each switch element is controlled to be turned on and off on the basis of the adjustment data, then the first constant current source for flowing a current through the resistor element and the second constant current source for flowing a current out of the resistor element are set. Thus, the quantity of adjustment and the increase or decrease of the reference voltage are set according to the adjustment data.
Furthermore, in one embodiment, the reference voltage producing circuit produces gamma-corrected reference voltages, and
the adjustment circuits are gamma correction adjustment circuits for adjusting the gamma-corrected reference voltages.
According to the above-mentioned construction, the reference voltage that is once gamma-corrected is further adjusted in accordance with the liquid crystal material and the LCD panel characteristics. Therefore, the reference voltage adjusted more correctly to the liquid crystal material and the LCD panel characteristics can be generated.
According to the second aspect of the present invention, there is provided an LCD drive unit comprising the gray scale-display reference voltage generating circuit.
According to the above-mentioned construction, the gray scale display reference voltage generating circuit can adjust each of the plurality of levels of reference voltages produced by the reference voltage producing circuit on the basis of the adjustment data. Therefore, by externally supplying the adjustment data, the reference voltages are simply adjusted in accordance with the liquid crystal material and the LCD panel characteristics without remaking the LCD drive unit.